1. Field of the Invention
The present invention relates to a control device for an internal combustion engine.
2. Description of the Related Art
As intake-side valve train mechanism of an internal combustion engine, so-called cam switching type valve train mechanism (or valve gear), includes a plurality of cams with different valve lift characteristics and acts to switch the cams by selecting one of the cams and drives an intake valve.
In an operation, the cam switching type valve train mechanism needs to switch ignition timing simultaneously with switching from one cam to another.
For this reason, there is known a control device for an internal combustion engine which controls ignition timing so as to have an intermediate value as a point of compromise without giving significant effect on operability of any cam during a cam switching period (see, e.g., Patent Document 1: Japanese Patent Laid-Open No. 2004-100547).
There is also known a so-called “three-dimensional cam” having a cross-sectional shape which changes continuously in a rotation axis direction. The three-dimensional cam includes a plurality of plane cam shape portions and a transitional shape portion. The plural cam shape portions have valve lift characteristics respectively different from each other and have respective cam faces parallel to a rotation axis of a camshaft. The transitional shape portion is located between the adjacent plane cam shape portions and has a curved cam face which connects the cam faces of the adjacent plane cam shape portions so as to be continuous with the cam faces. Herein, it is further to be noted that the term “plane cam shape” means a cam shape, which actually has a cubic (curved-) shape parallel to a camshaft, and in a sectional shape along the cam shaft, is parallel to the cam shaft compared with “transitional shape” which is oblique to the cam shaft.
As like as cams of a conventional valve train mechanism, in the three-dimensional cam, torque curves of one plane cam shape portion and another plane cam shape portion under a same intake negative pressure do not cross each other. That is, the plane cam shape portions have widely different cam profiles.
The wide differences among the plane cam shape portions cause a significant change in engine power at the time of switching between plane cam shape portions, which may give discomfort feeling in operation of a vehicle.